The present invention is concerned with a metal forming process and, in particular, a metal forming process involving spray deposition of atomized metal onto a substrate. (The term metal, as used herein, encompasses pure metal, metal alloys and composites having metal matrices, and ceramics).
Spray deposited products are made by an incremental process in which the product is built up from successive layers of deposit. In most cases this means that the last deposited layer is laid down on a cooler earlier deposit. This generally leads to the build up of internal stresses because of volumetric changes occurring during cooling; these internal stresses may lead to distortion or cracking of the product.
In order to eliminate distortion and cracking it is necessary to develop a suitable spray strategy and a means of controlling the volumetric changes that occur during cooling of the successive deposited layers.
Volumetric changes occur in three regions of a solidifying metal. Firstly, in the region above the liquidus, volumetric changes occur as cooling takes place, but no stresses develop because of the flow of liquid. Secondly, in the region between the liquidus and solidus, volume changes do occur but internal stresses do not develop on cooling until only a small fraction of liquid remains, in which case super-solidus cracking may occur.
The third region in which internal stresses (compressive or tensile) may develop on cooling is below the solidus temperature; these stresses may result in distortion or cracking. Here two phenomena are important:
(a) further shrinkage in most metals directly related to their coefficients of thermal expansion, and PA0 (b) phase changes occurring as the temperature falls, or the inclusion of reaction products formed by reaction with the atomizing gas, for example, leading to volumetric changes which are superimposed on (a). PA0 (i) depositing atomized metal onto a substrate so as to cause at least partial solidification of the deposited metal; PA0 (ii) depositing further atomized metal onto said partially solidified deposited metal on said substrate; and PA0 (iii) allowing the metal deposited onto said partially solidified deposited metal to fully solidify on the substrate; PA0 (a) On sprayforming, metal droplets are undercooled before the first solid is formed. That is to say that in the above example describing the previously expected behaviour of a 0.8% C steel, the nucleation of solid would not occur at the equilibrium solidus temperature. In fact this nucleation would be delayed--maybe considerably delayed--until some lower temperature. The contraction stresses developed in the austenite would then be reduced, because they would result only from cooling from the final nucleation temperature down to the martensitic transformation temperature. If, for example, nucleation first occurred at 805.degree. C. instead of 1400.degree. C., then the linear contraction would be precisely half that calculated previously in the example, leading to a volumetric contraction calculated as before of .about.2.2%; and the formation of approximately 51% martensite at the martensitic transformation temperature would be sufficient to compensate for the thermal contraction stresses in the austenite. PA0 (b) With regard to the observation that martensite appears to form, in practice, in samples of. 0.8% C steel deposited under conditions where the steady state deposition temperature is above the martensitic transformation temperature, this can also be explained by the non-equilibrium nature of the process. In hindsight it is entirely possible that individual droplets would cool below the martensitic transformation temperature before recalescing to a higher temperature on the substrate due to the evolution of latent heat. The conditions that would lead to this are not readily predictable "a priori", but the practical observations made in executing various embodiments of the current invention would point strongly towards the operation of this mechanism.
Both of these phenomena can affect the build up of internal stresses, and therefore distortion of the product with, in extreme cases, cracking or spalling.
We have now developed a spray deposition metal forming process in which this build-up of internal stresses resulting from thermal contraction or shrinkage on solidification or cooling can be matched or offset by other volumetric changes taking place in the deposit.